spongy bone

**Structure** - Network of interconnecting plates/rods (trabeculae) oriented along lines of mechanical stress (Wolff's law). Spaces between trabeculae contain red (hematopoietic) or yellow (fatty) marrow. - Trabeculae are made of lamellar bone. They have no Haversian systems (osteons), and nutrients reach osteocytes by diffusion through canaliculi from the marrow space. - Compact (cortical) bone is the dense outer shell (~80% of skeletal mass). It has osteons, Haversian canals, and Volkmann canals.

**Location** - Vertebral bodies, pelvis (iliac crest), sternum, ribs, and the epiphyses and metaphyses of long bones. - The medullary cavity of the diaphysis is mostly marrow, with little trabecular bone.

**Function** - Shock absorption and load distribution (e.g., vertebral bodies and femoral neck). - Houses red marrow, the site of hematopoiesis. In adults this is mainly the axial skeleton and proximal femur/humerus. - Mineral reservoir (Ca²⁺, PO₄³⁻). It has about 8× the surface area of cortical bone, so it is remodeled faster (higher turnover).

**Clinical correlations (high-yield)** - **Osteoporosis**: Loss of trabecular bone comes first, especially in postmenopausal women (estrogen deficiency increases osteoclast activity). Typical fractures are vertebral compression fractures, distal radius (Colles) fractures, and femoral neck fractures. Serum Ca²⁺, PO₄³⁻, PTH, and ALP are normal. Diagnosis is by DEXA (T-score ≤ −2.5). Treatment is bisphosphonates (inhibit osteoclasts, risk of osteonecrosis of the jaw and atypical femoral fractures), plus calcium and vitamin D, with denosumab or teriparatide as alternatives. - **Hyperparathyroidism**: Primary disease preferentially affects cortical bone (e.g., distal radius), with subperiosteal resorption and osteitis fibrosa cystica. Trabecular bone is relatively preserved. - **Bone marrow biopsy/aspiration**: Taken from the posterior iliac crest, which is rich in trabecular bone and red marrow. - **Osteopetrosis**: Defective osteoclasts (carbonic anhydrase II or CLCN7 mutations) lead to dense but brittle bone, a narrowed marrow space, pancytopenia, extramedullary hematopoiesis, and cranial nerve compression. - **Osteomyelitis**: In children, it often starts in the metaphysis (slow, sluggish blood flow). Staph aureus is the most common cause, and Salmonella is classic in sickle cell disease. - **Avascular necrosis**: Often involves subchondral trabecular bone in the femoral head. - **Paget disease**: Disordered remodeling produces a "mosaic" pattern of lamellar bone with elevated ALP and normal Ca²⁺/PO₄³⁻.

**Mnemonic**: Trabecular = "T for turnover and tension lines." It is the first site of osteoporotic loss and is most responsive to remodeling.